Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cwjcc!tut.cis.ohio-state.edu!rutgers!att!mtunb!dmt
From: dmt@mtunb.ATT.COM (Dave Tutelman)
Newsgroups: comp.sys.ibm.pc
Subject: Re: How Turbo C Handles Null Pointer Assignments
Summary: ... and a tool to help debug such problems.
Message-ID: <1497@mtunb.ATT.COM>
Date: 11 May 89 11:25:06 GMT
References: <4178@sybase.sybase.com> <1778@wasatch.utah.edu>
Reply-To: dmt@mtunb.UUCP (Dave Tutelman)
Organization: AT&T Bell Labs - Middletown, NJ
Lines: 119

In article <1778@wasatch.utah.edu> jacobs%cmos.utah.edu@wasatch.utah.edu (Steven R. Jacobs) writes:
>In article <4178@sybase.sybase.com> forrest@sybase.com writes:
>>... [ Turbo C checks ] ...
>>the memory area starting with address 0 and continuing through
>>where the message is stored that you see when you run Turbo C.
>
>This "checksum" is actually the Turbo C copyright string.  If it
>gets tromped on, the "Null Pointer Assignment" message is given.

Both Microsoft C and Turbo C detect Null Pointer Assignments this way.
(Maybe others do as well.)  The code below is a debugging tool I have
occasionally used to track down null pointer assignments in C programs.
It is based on the same principle.
It has worked in Microsoft C 4.x and 5.0, and Turbo C 1.0 and 2.0.
It remains independent of which compiler/version you use because it
initializes its reference string at run time (based on the initial
contents of the null-pointer area).

Enjoy!
+---------------------------------------------------------------+
|    Dave Tutelman						|
|    Physical - AT&T Bell Labs  -  Middletown, NJ		|
|    Logical -  ...att!mtunb!dmt				|
|    Audible -  (201) 957 6583					|
+---------------------------------------------------------------+

/*************************************************************
 *
 *	NULLPTR - Dave Tutelman - March 1988
 *
 *	Test for a null pointer violation.
 *	Complain and quit if found.
 *
 *	nullptr (string)
 *		char *string;
 *
 *	FIRST CALL - string should be empty (""), to initialize.
 *	SUBSEQUENT CALLS - if a null pointer violation is found,
 *		the program displays (1) the string, and (2) the
 *		null pointer area violations,  then quits.
 *
 *	As distributed, this file contains a sample main program to
 *	   (1) demonstrate the use of nullptr(), and
 *	   (2) test its effectiveness in your environment.
 *	To use nullpr() in your own program, recompile after commenting out
 *	the "#define SAMPLE" line.
 */

#define SAMPLE

#include <stdio.h>

#define	NULLVECSIZE	80

static char nullvector[NULLVECSIZE];

nullptr (s)
	char *s;
{
	int	i;
	char	*p;

	if (!strlen (s)) {	/* initialize nullvector */
		for (i=0, p=NULL; i<NULLVECSIZE; i++,p++)
			nullvector [i] = *p;
	}

	else {		/* check to see if null vector has changed */
		for (i=0, p=NULL; i<NULLVECSIZE; i++,p++)
			if (nullvector [i] != *p)  quit (s);
	}
}


quit (s)
	char *s;
{
	int	i;
	char	*p;

	printf ("\nNULL POINTER VIOLATION DETECTED!\n");
	printf ("%s\n", s);

	printf ("\nOriginal and corrupted values of vector\n");
	printf (" n    Hex    Char\n");
	for (i=0, p=NULL; i<NULLVECSIZE; i++,p++)
		if (nullvector[i] != *p)
			printf ("%2d   %2X %2X   %c %c\n", i,
				nullvector[i], *p, nullvector[i], *p);

	exit (-1);
}


#ifdef SAMPLE
/******************************************************************
 *
 *	Sample main program for nullptr ()
 */

main ()
{
	char	*p;

	nullptr ("");		/* initialize */

	nullptr ("Nothing yet!");

	p = NULL;  p +=4;
	*p = 'V';	/* violate the null ptr area */
	p += 20;
	*p = 'I';	/* and again */
	nullptr ("This one should catch it!");

	printf ("Shouldn't ever get here - printf\n");
	nullptr ("Shouldn't ever get here - nullprt\n");
}

#endif